I. Field of the Invention
The invention relates to the determination of the content of specified oxygenated components in a variety of liquids, particularly the concentrations of various alcohols and ethers in hydrocarbon liquids.
Recent U.S. government environmental legislation has resulted in stringent regulatory agency guidelines for product makeup for the chemical and petroleum industries. The guidelines require light control of the chemical composition of these industries' products particularly the composition of gasoline. The oxygenate content of gasoline has received particular attention, with the requirement that reformulated gasolines contain between 2.0 and 2.7 percent by weight of oxygen it is possible to estimate the volume percentage of oxygenate, or the total weight percentage of oxygen in blended gasolines, based on known or estimated blend compositions, level and purity of oxygenate addition. However, it is not always possible to obtain the information required for an accurate calculation. Because it is likely that governmental regulation of the chemical composition of various products and fuels will increase in the future, efficient chemical, refined, and blending operations will require improved analytical procedures to insure compliance with the guidelines.
II. Description of the Prior Art
Prior patents related to the analysis of aromatics in hydrocarbon streams include U.S. Pat. No. 4,963,745 to Maggard, issued Oct. 16, 1990; U.S. Pat. No. 5,223,714 to Maggard, issued Jun. 29, 1993, U.S. Pat. No. 5,243,546 to Maggard, issued Sep. 7, 1993; U.S. Pat. No. 5,145,785 to Maggard and Welch, issued Sep. 8, 1992; international application WO 93/24823, published Dec. 9, 1993.
U.S. Pat. No. 5,349,188 to Maggard, issued Sep. 20, 1994, teaches the determination of octane generally, and U.S. Pat. No. 5,349,189 to Maggard, issued Sep. 20, 1994, teaches the determination of hydrocarbon groups by group type analysis.
Prior art teachings of the determination of oxygenated species can be found in prior literature and patents. A preferred technique is gas liquid chromatography with Oxygen Flame Ionization Detection (OFID), wherein a sample is injected into a partitioning column swept by an elutriating inert gas, e.g., 5% hydrogen in helium. Separated oxygenates in effluent from the partitioning column are converted to carbon monoxide by a cracking reactor, and then to methane by a methanizer. A flame ionization detector detects the several methane bands so produced from each of the oxygenates. The elapsed time for elutriation through the system is measured for the methane band representing each oxygenate. Non-oxygenated hydrocarbons do not interfere with this analysis because they are converted to elemental carbon and deposited on the catalyst contained in the cracker. The OFID procedure and apparatus used for this analysis are illustrated hereinafter in Example 5 and FIG. 4.
Conventionally, the percentages of each of the individual oxygenated compounds is determined in weight percent total oxygen, and volume percent of each oxygenate as required. An example of this procedure is that taught by Wasson ECE Instrumentation, Inc. (1305 Duff Drive Suite 7, Fort Collins, Colo. 80524, Operations Manual Serial Number 930931). Although precise, gas chromatography is time consuming and labor intensive, and the considerable lag time involved can result in unacceptable cost when productions errors occur.
Recently, near-infrared (NIR) spectrophotometric analysis has been used to perform oxygenate analysis. U.S. Pat. No. 5,362,965 to Maggard teaches the determination of oxygenate content in gasolines and other hydrocarbon fuels, with selection of wavelength ranges and data preprocessing to minimize the temperature dependence of the calibrations.
As far back as 1948, Raman spectroscopy was considered for determination of aromatics content in hydrocarbon mixtures (U.S. Pat. No. 2,527,121). For a variety of reasons, however, extensive use of this procedure as a quantitative technique has not occurred to the degree of mid-IR or near-IR absorbance/reflectance spectroscopic methods. One reason for this may be that a significant limitation of Raman spectroscopy has been the presence of interfering fluorescence signals (with the exception of aviation fuel) due to excitation by visible lasers.
Recently, FT-Raman spectrometers have been developed which eliminate the fluorescence problem in many cases by exciting in the NIR spectral region. This capability has sparked renewed interest in the use of Raman spectroscopy in the analysis of petroleum samples. For example, Shope, Vickers and Mann (Appl. Spectrosc., 1988, 42, 468) have demonstrated that when analytes are present in liquid mixtures as minor components, Raman spectroscopy is a viable quantitative technique. Using NIR-FT-Raman spectroscopy in combination with multivariate analysis techniques, Scasholtz, Archibald, Lorber and Kowalski (Appl. Spectrosc., 1989, 43, 1067) have demonstrated that quantitative analysis of percentage of fuel composition is possible for liquid fuel mixtures of unleaded gasoline, super-unleaded gasoline, and diesel fuels. In addition, Williams and co-workers (Anal. Chem., 1990, 62, 2553) have shown that NIR-FT-Raman spectroscopy in combination with multivariate statistics can be used to determine gas oil octane number and octane index. Chung, Clarke, and others have shown that Raman spectroscopy can be used in the quantitative analysis of aviation fuel in the determination of general hydrocarbon makeup, aromatic components, and additives (Appl. Spectrosc., 1991, 45, 1527; J. of Raman Spectrosc., 1991, 22, 79).
Recently, Allred and McCreery described an NIR dispersive Raman instrument utilizing a GaAIAs NIR diode laser, a single-stage imaging spectrograph, CCD detection, and a fiber-optic probe (Appl. Spectrosc., 1990, 44, 1229; Appl. Spectrosc., 1993, 46, 262) for benzene and KNO.sub.3 analysis. More recently, Cooper and co-workers have demonstrated (Spectrochimica Acta, 1994, 50A, 567) that low-cost CCD detection is feasible for remote fiber-optic Raman detection. While NIR technique is a viable analytical method for the majority of oxygenated species, the spectral similarity of the oxygenates in the NIR absorbance region make quantitation of individual compounds difficult with NIR when more than one compound is present in significant concentrations. Accordingly, there has remained and for a more effective procedure tier measurement of oxygenates in a variety of liquids, particularly in fuels. The invention addresses this need.